1. Field of the Invention
The present invention relates to a method and apparatus for energizing a thermal head of a heat transfer or a heat sensitive thermal printer to print the printing data line by line.
2. Description of the Prior Art
A prior art energizing apparatus of a thermal head of a thermal printer is, as shown in FIG. 1, connected to external equipment such as a personal computer or the like which produces the printing data, for example, image data representing a wire frame pattern to be printed. The energizing apparatus includes an interface circuit (hereinafter referred to as an I/F circuit) 1 such as a Centronics interface or the like which receives the image data, a computer (referred to as a CPU) 2 for controlling the operation of the thermal printer as a whole, a random access memory (referred to as a RAM) 3 for a work area, a read only memory (referred to as a ROM) 4 for storing a program, a manipulation circuit 5, a thermal head S having a shift register 6a, latch circuit 6b, a driving circuit 7a of the thermal head, and a heating unit 7b including heating elements, a driver circuit 8, a paper feeding pulse motor 9, a transfer ribbon take-up pulse motor 10, and a solenoid 11 for pressing the thermal head against a ribbon and printing paper.
The image data input from the external equipment through the I/F circuit 1 is supplied via the CPU 2 to the shift register 6a for each line sequentially and stored therein.
Thereafter, the image data in the shift register 6a is transferred to the latch circuit 6b by applying a latch signal. Then, a common signal is delivered to the driving circuit 7a from the CPU 2 for a time depending on the temperature of the thermal head S to supply a current to predetermined heating elements of the heating unit 7b to achieve printing.
In this case, in the thermal printer, when the electric power is to be supplied to the heating elements for one line of dots which amount to several thousands of dots, a power source of a large capacity is required. However, to avoid this, the one line of dots or heating elements are divided into a certain number of blocks and the energization of the heating elements is carried out for each block as a unit.
Such a block is called a common, and the printing of one line of dots is achieved by sequentially supplying a common signal from the CPU 2. On the other hand, various driving commands are input to the driver circuit 8 from the CPU 2 via the manipulation circuit 5, and the paper feeding by the pulse motor 9, and the taking-up of a transfer ribbon by the pulse motor 10 are performed, and at the same time, by exciting the solenoid 11, the printing of the image data is performed in accordance with the type of the thermal printer either the heat sensitive type or the thermal transfer type.
In the prior art energizing apparatus, among the heating elements of the thermal head, each of the heating elements which performs printing is supplied with a current for a fixed time, whereas each of the heating elements which does not perform the printing is not supplied with current. In this respect, in some prior art apparatus, the preheating is performed, for example, the printing head is maintained at a constant temperature independent of the printing data, or a separate heating head is provided separately from the printing head at a position several lines preceding the present printing line. Thus, the problem is involved in that the control of the preheating can not be achieved in accordance with the surrounding contents of the data to be printed, and the construction of hardware is complicated.
Accordingly, in printing a wire frame pattern in which a printing portion appears for the first time after a succession of non-printing portions for relatively a long time, for example, a grid pattern consisting of vertical line and horizontal line as shown in FIGS. 2A to 2C, the heating elements will be cooled before they reach the printing portion. As a result, a thin or broken portion will appear in the printed portion.
Generally, a relationship between the heating time of the heating element and the actual effect of printing on a printing paper is illustrated as shown in FIG. 3. When the heating time is shorter than T2, a non-printed area appears on the printing paper, and when the heating time is between T2 and T3, an intermediate area is produced in which printing or non-printing is effected depending on an environmental temperature. Furthermore, when the heating time exceeds T3, a printed area is produced. Hereinafter, the heating time is represented by the scale in FIG. 3 for the sake of explanation, for example, T2 is represented by "2", T3 by "3", a maximum heating time of a normal printing area by "6", and a maximum heating time for printing a pattern line portion to correct heating by additionaly heating thereby to emphasize in the present invention is represented by "7". Furthermore, a heating time T1 for preheating in a non-printing area is represented by 1".
The thin or broken printed portion is caused in various cases. However, in the present invention, the following cases are the objects for preventing such a thin or broken printed portion.
(i) The thin or broken portion caused in a horizontal pattern line when a grid pattern consisting of vertical and horizontal pattern lines is to be printed (FIG. 2A).
(ii) The thin or broken portion caused in the vertical pattern line when an adjacent area to the horizontal pattern line is preheated to prevent the thin portion in the horizontal pattern line in the case of (i) (FIG. 2B).
(iii) The thin or broken portion appearing in a first dot column and a first dot row in the printing of broad vertical and horizontal pattern lines each having a width of two dots (FIG. 2C).
In the cases (i) and (ii) mentioned above, the causes of the occurrence of the thin or broken printed portion are as follows.
In printing of the vertical pattern line, when the printing is performed line by line, the amount of heat supplied to the heating element for printing the previous line is accumulated so that it is summed to the heat for the printing of the present line. As a result, the amount of heat required for the printing is always obtained, and no thin or broken printed portion is usually. However, in the case of printing the horizontal pattern line, the printing is performed after passing through a large non-printed portion. Thus, the heating element is cooled and since there is no accumulated heat as mentioned above, the amount of heat required for printing is insufficient. As a result, even when a current is supplied to the heating element for the same period of time as in other printing portions, the thin printed portion will occur (FIG. 2A).
Furthermore, in order to prevent the occurrence of the thin or broken portion in the horizontal pattern line, if the correction for heating, that is, preheating for the non-printing portion is performed as in a first embodiment of the invention (described later), since this preheating is not performed for just lateral adjacent portions of the vertical pattern line to be printed, the printed vertical pattern line looks thin in contrast to the printed horizontal pattern line which has been made clear. That is, the vertical pattern line becomes thin relative to the horizontal pattern line (FIG. 2B).